The expression of the genes coding for cholecystokinin (CCK), enkephalin and tyrosine hydroxylase (TH) was studied pre- and post-natally in the rat brain by using the corresponding specific cDNA probes to quantify mRNA. In addition, specific radioimmunoassays were used to measure CCK-immunoreactive peptides (CCK-IR) and enkephalin immunoreactive peptides. For the study on developmental expression of the tyrosine hydroxylase gene, mRNA levels were measured comparatively in the substantia nigra and the locus coeruleus and the levels of noradrenaline, dopamine and their metabolites were measured at the same time.